I was reading about this the other day, installation is still a bit tricky and functionaility is still limited, but you should be able to have mythtv frontend on at least the N800. You effectively need to setup gstreamer which streams the video/tv for you at a lower resolution that the nokia device can handle.
The linuxMCE orbiter will currently only act as an orbiter, allowing you to control things. This works really well but sort out your wi-fi first. I was having a lot of problems with it originally which turned out to primarily be issues with the wireless access point.